Record-breaking cosmic mirage

photo: This picture, taken by the Subaru Telescope on the summit of Mauna Kea in Hawaii, shows four images of the same quasar (the four white dots in the center). The quasar is almost 10 billion light-years from us and its light has been split into four by the gravitational influence of a foreground cluster of galaxies 6.2 billion light-years from us. Some of the galaxies of the cluster appear as yellow dots in the image. Sloan Digital Sky Survey


January 1, 2004

A widely lensed quasar reveals the presence of dark matter.Having something block your view when taking a photo can be a real pain — except perhaps when studying cosmic mirages and dark matter.

Astronomers have discovered a gravitationally lensed quasar located more than 10 billion light-years away that is shedding new light on dark matter in the universe. Parked behind a massive cluster of galaxies, the quasar's feeble light has been bent and split into four distorted images that have the largest angular separation ever found. According to the discovery team, this wide-angle effect is evidence that invisible cold dark matter dominates the foreground cluster and is responsible for the record-breaking quadruple mirage.

Since 1979, more than 80 gravitationally lensed quasars have been cataloged, however none were found to have separations of more than 7 arcseconds. Despite theoretical models that predicted larger splitting of quasar images, numerous searches had come up empty — until now.

Mining the colossal database of over 30,000 quasars from the Sloan Digital Sky Survey (SDSS), an international team of astronomers led by Naoisha Inada and Masamune Oguri from the University of Tokyo pinpointed SDSS J1004+4112 in the constellation Leo Minor.Using the Subaru Telescope on the summit of Mauna Kea, Hawaii, the group managed to identify four individually split quasar images separated by 14.62 arcseconds — more than twice as large as the previous record-holding lensed quasar.



photo: This 2.5-meter telescope is the main workhorse of the Sloan Digital Sky Survey. Its box-shaped structure protects it against the wind. SDSS Collaboration.

"Additional observations obtained at the Subaru 8.2-meter Telescope and Keck Telescope confirmed that this system is indeed a gravitational lens," explains lead author Inada. "Quasars split this much by gravitational lensing are predicted to be very rare, and thus can only be discovered in very large surveys like the SDSS."

First predicted by Einstein more than six decades ago, gravitational lensing occurs when the gravity from a massive foreground object bends and amplifies the light from a more distant object, as seen from Earth. Astronomers have been using this giant magnifying-lens effect to bring into view quasars and galaxies that otherwise would be too faint to detect. Some lensed quasars produce multiple images including, in rare cases (if the alignment is perfect), rings around the lensing galaxies.In the December 18 edition of Nature, the team argues that the quadruple lensing effect at J1004+4112 is caused by the gravitational influence of a cluster of galaxies about 6.2 billion light-years away. They believe that because the visible mass of this cluster cannot account for the observed 14.62-arcsecond separation, high concentrations of intervening material in the form of unseen cold dark matter must be causing this unprecedented wide splitting.



photo: In a gravitational lens, a foreground galaxy (here shown as a red spiral) causes light (dashed lines) from a background quasar to bend. As a result, an observer may see multiple quasars instead of just one. In most cases the quasar images are only offset by an arcsecond, but occasionally, as is shown here, the offset is much larger. Astronomy.com: Pamela L. Gay

Oguri added: "Discovering one such wide gravitational lens out of over 30,000 SDSS quasars surveyed to date is perfectly consistent with theoretical expectations of models in which the universe is dominated by cold dark matter. This offers additional strong evidence for such models."

The authors expect many more such wide-angle lensed quasars will be encountered and that they will become powerful tools in the study of the distribution of dark matter in the universe. "The gravitational lens we have discovered will provide an ideal laboratory to explore the relation between visible objects and invisible dark matter in the universe," adds Oguri.